Unlike traditional film cameras that use film to capture and store an image, digital cameras use solid-state microelectronic image sensors to capture an image and use digital memory to store the image. The microelectronic image sensors are small silicon chips (also referred to as integrated circuits or ICs), or die. The microelectronic image sensors are variously referred to as imager chips or ICs, image capture chips or ICs, imager die or microelectronic imagers. An imager chip contains thousands to millions of photosensitive detectors called photosites. The combination of a photosite and its circuitry is referred to as a pixel. When the shutter (mechanical and/or electrical) is open or enabled, each photosite records the intensity or brightness of the incident light by accumulating a charge; the more light, the higher the charge. The brightness and/or color data for a corresponding pixel of the captured image is subsequently read out from the capture circuitry to digitization circuitry and then to digital storage circuitry. Digitization can be accomplished on the imager chip (for example within the pixel, at each array column, or after row/column multiplexing) or accomplished with analog-to-digital circuitry external to the imager circuitry. The digital values representing brightness and color can then be used to reconstruct the captured image on a variety of display mechanisms or ink printed paper.
Microelectronic imagers are used in digital cameras, cell phones, Personal Digital Assistants (PDAs), other wired and wireless devices with picture taking (image capture) capabilities, and many other imaging applications. The market for microelectronic imagers has been steadily increasing as they become smaller and produce better quality images with higher pixel counts. In order to reduce manufacturing cost and size of the entire image sensor, new approaches are required to reduce optics complexity, improve optical performance, simplify and automate optics alignment, and reduce overall component count and size in the final image sensor assembly.
Microelectronic sensors include integrated circuits such as Charged Coupled Device (CCD) image sensors or Complementary Metal-Oxide Semiconductor (CMOS) image sensors. CCD image sensors have been widely used in digital cameras because of their high performance. CMOS image sensors are displacing the CCD in many applications because performance is rapidly improving comparable to the CCD, and the high yields of the CMOS fabrication process enable low production costs for each imager chip. CMOS image sensors can provide these advantages because they are manufactured using technology and equipment developed for fabricating standard integrated circuit semiconductor devices. CMOS image sensors, as well as CCD image sensors, are packaged to protect the delicate components, interface with optical components and provide external electrical contacts.
FIG. 1 is a cross-sectional view of a conventional microelectronic imager module 1 with a conventional package and associated optics. The imager module 1 includes an integrated circuit die 10, an interposer substrate 20 attached to the die 10, and a housing 30 attached to the interposer substrate 20. The housing 30 surrounds the periphery of the imager die 10 and has an opening 32. The imager module 1 also includes an optically transparent cover 40 over the die 10.
The integrated circuit die 10 includes an image sensor region and associated circuitry 12 and a number of bond-pads 14 electrically coupled to the electrical circuitry 12. The interposer substrate 20 has a plurality of wire bond-pads 22, a plurality of bump/solder-pads 24, and traces 26 electrically coupling bond-pads 22 to corresponding bump/solder-pads 24. The bump/solder-pads 24 are arranged in an array for surface mounting the imager 1 to a board or module of another device. The wire bond-pads 14 on the die 10 are electrically coupled to the wire bond-pads 22 on the interposer substrate 20 by wire-bonds 28 to provide electrical pathways between the wire bond-pads 14 and the bump/solder-pads 24.
The imager module 1 also has an optics unit including a support 50 attached to the housing 30 and a barrel 60 adjustably attached to the support 50. The support 50 can include internal threads 52, and the barrel 60 can include external threads 62 engaged with the threads 52. The optics unit also includes an assembly of lenses 70 carried by the barrel 60. The optical focus is achieved by moving all the lenses in unison towards the imaging sensor until optimal performance is achieved.
One problem with packaging a conventional microelectronic imager conventionally as shown in FIG. 1 is that the resultant imaging module has a relatively large footprint. The footprint of the imager module 1 for example is the surface area of the bottom of the interposer substrate 20. This is typically much larger than the surface area of the die 10 and can be a limiting factor in the design and marketability of picture cell phones or PDAs because these devices are continually shrinking to be more portable. Therefore, there is a need to provide microelectronic imager modules with smaller footprints.
Another problem with packaging a conventional microelectronic imager is the complexity of the optical assembly and focus mechanism. The optical assembly 70 typically has a diameter significantly larger than the image sensor region 12. The optical assembly is connected to a lens barrel 60 that adds additional diameter size to the imager footprint. The lens barrel 60 has threads 62 that mate with threads 52 on the support 50. These sets of threads align the optics to the image sensor and provided movement in the z-dimension to obtain accurate optical focus and sharpness of image. All the precision aligned optic lenses in the assembly 70 are displaced together in the z-direction to adjust the back focal length and focus the imager. The combination of optical assembly 70, barrel 60 and support 50 further increases the diameter size and module footprint. The use of threads and barrel rotation, R, with respect to the support 50 to focus the optics is difficult to implement in an automated assembly of the imager. The thread movement is also a source of particles than can eventually reside over the imaging area where they may degrade image quality. The requirement for threads also increases cost of the module. Alignment of the image capture components can be difficult, particularly in small cameras (e.g., cameras in mobile telephones) because multiple devices are mounted on the interposer substrate and the tolerances accumulate to reduce the precision with which the image capture device components can be aligned.
Another disadvantage of the conventional microelectronic imager is the relatively poor imaging performance, specifically modulation transfer function (MTF) sensitivity to z-alignment accuracy of the optical assembly 70 relative to the image sensor region 12. FIG. 2 is a diagram that illustrates degradation in MTF variation with change of back focal length (BFL) (40-micron z-axis lens group travel) for a typical three-element optical design under the prior art. In FIG. 2 the MTF rapidly degrades, and in order to achieve MTF better than 0.4 the optical lens group needs to be aligned to within about ±10 microns. Alignment accuracy of 12 of ±10 microns between the optical assembly 70 and image sensor region is difficult to achieve.
What is needed, therefore, is an imager module that reduces optical complexity while maintaining high imaging performance, alleviates mechanical alignment problems, allows automated assembly, requires fewer components, provides smaller overall imager module footprint and less electrical interfaces than prior art solutions. What is also needed is an imager module that requires fewer manufacturing steps, shorter assembly time and lower cost as compared to prior imager modules.